Hans J. Eysenck developed descriptions of personality based on what he saw as physiological causes of personality to behavioral effects. He developed questionnaires integrating the concepts of extroversion-introversion, neuroticism and psychoticism to develop his 3-factor model (Module 3: Constitutional approach.). This model is known as the PEN model. A five-factor model is an approach created from factors that many personality researchers have discovered in their own studies. Costa and McCrae are very well known for the OCEAN model. OCEAN is an acronym for openness to experience, conscientiousness, extraversion, agreeableness and neuroticism (Module 3: Constitutional approach.). "Personality structure among prisoners: how valid is the five-factor model and can it support Eysenck's theory of crime?" is a study that was created following another study carried out before it. The previous study was based on offender research using a sample. Both samples included Category B inmates. All study participants completed the International Personality Item Pool Big Five Factor Markers (IPIP) to access their personality. The IPIP is made up of 50 items with responses based on a scale of 1 to 5. This test is accessible to everyone free of charge and is very broad. Due to its ease of access and breadth, it may not be the most reliable in an uncontrolled environment. Participants were also informed beforehand of the nature and purpose of the study, which carries with it the possibility of lying or exaggeration. Other measures that could have been included in the study would have been a wider range of participant samples, the use of participants who did not voluntarily agree to participate, a wider age range among participants and adding the inverse.